The present invention relates to the field of auto-darkening eye protection devices, such as welding helmets having a shutter (or lens) assembly that automatically darkens upon the detection of a welding arc. A photosensitive device such as a photodiode or a phototransistor may be used to sense the intensity of light incident on the area of the shutter assembly so as to provide an indication to the circuitry controlling the shutter assembly that the shutter assembly needs to be driven to either a dark state or a clear state. If a welding arc is present, the welding helmet protects the eyes of the welder from any danger caused by the intensity of the welding arc by driving the shutter assembly to a dark state, thereby decreasing the amount of energy passing through the lens to the welder's eyes. U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, U.S. Pat. Nos. 5,248,880, 5,252,817, 5,347,383, 5,533,206, 5,751,258, 5,959,705, 6,067,129, and 6,070,264 each disclose various shutter assemblies and liquid crystal driver electronics that can be used in conjunction with the present invention. The disclosures of these above-mentioned patents are hereby incorporated in their entireties by reference.
Commonly-owned U.S. Pat. No. 5,347,383 discloses a driving circuit for a liquid crystal shutter. The sensor circuitry of this invention utilizes a photodiode to detect the occurrence of welding. This sensor circuitry also utilizes a comparator to compare the sensed light signal with a threshold value to determine whether the shutter assembly should be driven to a dark or clear state. Additionally, the '383 patent discloses the use of a 9 V supply.
While the invention disclosed in this patent functioned for its intended purpose, a need was felt for an improvement in the power consumption by the sensor circuit. As incident light increases on a photodiode, the voltage across the photodiode will begin to saturate. To prevent the photodiode from saturating, a steadily increasing load must be put on the photodiode which leads to excessive power consumption.
To alleviate the excessive power consumption inherent in a photodiode-based sensor circuit, a phototransistor has been utilized as a weld sensor. The use of a phototransistor allows the use of feedback to bias the phototransistor so that less current is needed to keep the phototransistor in its operational mode. Commonly-owned U.S. Pat. Nos. 5,252,817, 5,248,880, 5,751,258, and 6,070,264 are illustrative of sensor circuits using phototransistors as weld sensors. Each of these patents discloses a sensor circuit wherein the output of the phototransistor is fed into a comparator. The comparator compares the phototransistor output with a threshold level. If the phototransistor output exceeds the threshold level, the drive circuitry is activated to darken the shutter assembly. If the phototransistor output does not exceed the threshold level, the drive circuitry operates the shutter assembly in a clear state. While the circuits disclosed in these patents utilize feedback to bias the phototransistor and avoid the excessive drawing of current, heavy loads were still needed. The circuits implementing such designs used voltage supplies ranging from 5.6 V to 9 V. Therefore, a need was still felt for a sensor circuit having improved power consumption characteristics.
Moreover, the phototransistors used in the prior art designs were metal can phototransistors. Metal can phototransistors are relatively big and bulky. Their size, height and relative difficulty in mounting serves as a limiting factor in the ability of designers to reduce the size of the units in which the sensor circuit is implemented. Thus, a need was felt to use a smaller and more compact phototransistor that is more easily mountable to a circuit board to produce a smaller, sleeker unit while still having the ability to maintain a constant signal level without excessive loading or the drawing of excessive current.
Additionally, the sensor circuits of the prior art produced an output voltage from the phototransistor in response to incident light intensity as seen in FIG. 3 of the '880 and '817 patents. As can be seen, low light intensities produce a steep rise in output of the phototransistor. Because of the power drain caused by the response of the phototransistor to low intensity incident light, it is desirable that the phototransistor be configured to minimize the phototransistor output signal when the sensor circuit is exposed to low intensity incident light. Thus, a need existed for a sensor circuit that provided greater attenuation in the response of the phototransistor to low intensity incident light.
While it is desirable to minimize the phototransistor output when the sensor circuit is exposed to low intensity incident light, the phototransistor still must be able to quickly increase its output in response to a transition from low intensity light to high intensity light, such as the light provided by a welding arc. Thus, an ever present need exists within the art to sharpen the rise provided by the phototransistor in response to sharp increases in light intensity.
Also, the sensor circuitry of the prior art used a comparator to correlate the sensed light signal with the desired shade level. The comparator compared the output of the phototransistor with a threshold voltage signal to determine whether the shutter assembly should be driven to a dark state or a clear state. This design required additional circuitry to set the threshold voltage level. This additional circuitry not only complicated circuit design, but also increased the drain on the power supply. Thus, a need was felt to simplify the sensor circuitry to provide a more power-efficient way of correlating the phototransistor output to the proper shade setting of the shutter assembly.